Balance the combustion reaction:
a C₂H₂ + b O₂ → c CO₂ + d H₂O
Count the atoms on both sides of the reaction:
• C : 2a = c
• H : 2a = 2d
• O : 2b = 2c + d
Let a = 1; then c = 2 and d = 1, and it follows that
2b = 2•2 + 1 = 5
so that b = 5/2, and the balanced reaction is
1 C₂H₂ + 5/2 O₂ → 2 CO₂ + 1 H₂O
or, with integer coefficients,
2 C₂H₂ + 5 O₂ → 4 CO₂ + 2 H₂O
C₂H₂ has a molar mass of about 26.038 g/mol, so we have a starting amount of
(64.0 g) (1/26.038 mol/g) ≈ 2.46 mol C₂H₂
According to the reaction, we produce H₂O in a 1-to-1 ratio with the C₂H₂ that is burned up, so we end up with about 2.46 mol H₂O.
